An inverter is used to generate from a direct current source the polyphase currents necessary for a polyphase rotating electrical machine to function.
An inverter generally includes switching elements forming a plurality of power arms each including two switching elements in a classic two-level bridge architecture.
The mid-point of a pair of switching elements of the same power arm is connected to a phase winding of the stator of the rotating electrical machine.
The switching elements are most often controlled by pulse width modulation (PWM) methods making it possible to apply to the rotating electrical machine voltages between phases of sinusoidal waveform.
The space vector modulation (SVM) process is very widely used and enables an improvement of 15% to be achieved over the conventional sinusoidal PWM method.
In the prior art, there are known other types of methods of controlling a polyphase inverter, notably a generalized discontinuous pulse width modulation (GDPWM) process described for example in the paper “A High-Performance Discontinuous PWM Algorithm”, A. M. Hava et al., IEEE Trans. on Industry Applications, vol. 34, no 5, September/October 1998, p. 1059-1071.
This control method blocks one of the power arms turn and turn about over an electric period.
Comparison of the various PWM methods in terms of efficiency has generated an abundant technical literature.
Studies oriented toward applications in motor vehicles include the paper “A Comparison between Pulse Width Modulation Strategies in terms of Power Losses in a Three-Phased Inverter—Application to a Starter Generator”, J. Hobraiche, J. P Vilain, M. Chemin, European Power Electronics Congress—Power Electronics and Motion Control, Riga, Latvia, September 2004.
The losses in the inverter are compared for the SVM and GDPWM processes as a function of the operating points of a starter generator.
Moreover, the increase in power of the onboard equipment in the vehicles is giving rise to new problems of electromagnetic compatibility (EMC), notably in relation to conducted interference.
To stabilize a supply voltage on the upstream side of the inverter, the current source generally includes a decoupling capacitor that makes it possible to filter the input current of the inverter that is subject to a high level of discontinuities because of the chopping effected by the switching elements.
French patent application FR2895598 in the name of VALEO EQUIPEMENTS ELECTRIQUES MOTEUR describes a specific PWM control method for controlling a polyphase inverter that enables both a reduction of the switching losses and a reduction of the rms current in the decoupling capacitor so as to reduce the ripple on the supply voltage.
The PWM control method described in this application applies to polyphase electrical loads in general, whereas most of the studies, as well as the papers cited above, are limited to three-phase electrical machines.
It is known that polyphase rotating electrical machines have advantages over three-phase machines in terms of reduced oscillation of the torque in motor mode and ease of elimination of harmonics in generator mode.
There is therefore a requirement for studies bearing on PWM control in terms of reduced switching losses and reduced conducted interference for polyphase machines.
However, the increase in the number of phases leads to an increase in the complexity of a control device of the machine and the inventors have concentrated on double three-phase machines that allow simplification of the control device compared to six-phase machines in general and that authorize an extrapolation of the results of studies carried out on three-phase machines, such as those published by J. Hobraiche et al. in the paper cited above.